Ratiometric fluorescent detection of Cu²⁺ based on dual-emission ZIF-8@rhodamine-B nanocomposites

Zeolitic imidazolate framework-8 (ZIF-8) loading rhodamine-B (ZIF-8@rhodamine-B) nanocomposites was proposed and used as ratiometric fluorescent sensor to detect copper(II) ion (Cu²⁺ ). Scanning electron microscopy, Fourier transform infrared spectroscopy, X-ray powder diffraction, nitrogen adsorption/desorption isotherms and fluorescence emission spectroscopy were employed to characterize the ZIF-8@rhodamine-B nanocomposites. The results showed the rhodamine-B was successfully assembled on ZIF-8 based on the π-π interaction and the hydrogen bond between the nitrogen atom of ZIF-8 and -COOH of rhodamine-B. The as-obtained ZIF-8@rhodamine-B nanocomposites were octahedron with size about 150-200 nm, had good water dispersion, and exhibited the characteristic fluorescence emission of ZIF-8 at 335 nm and rhodamine-B at 575 nm. The Cu²⁺ could quench fluorescence of ZIF-8 rather than rhodamine-B. The ZIF-8 not only acted as the template to assemble rhodamine-B, but also was employed as the signal fluorescence together with the fluorescence of rhodamine-B as the reference to construct a novel ratiometric fluorescent sensor to detect Cu²⁺ . The resulted ZIF-8@rhodamine-B nanocomposite fluorescence probe showed good linear range (68.4 nM to 125 μM) with a low detection limit (22.8 nM) for Cu²⁺ combined with good sensitivity and selectivity. The work also provides a better way to design ratiometric fluorescent sensors from ZIF-8 and other fluorescent molecules.